O/T Advice required from Heating Engineer

I am looking to get my combi boiler replaced. It is over 10 years old and starting to play up so looking for something more reliable and efficient. The current boiler has a flue that exits through our side gable end wall. There is a house that was built in the grounds of our former garden so technically the flue exits into their side lane at the side of their house. This is never used and there are no windows or doors on this side. we have no issues with our neighbour. I have just sought a quote from a company who on reflection cannot do the work without moving the boiler or major work with the flue. This seems OTT. Is this them just being fussy or they in line with regulations

Any advice gratefully received

Despite you saying the lane is never used they as installers have to assume people will pass through there so a flue at low level in a confined walk through is a risk.

It's the building regulations

Not sure if my pm worked. When are you free to take a look?